As one of methods of measuring a surface state of a measurement target, there is given a method including using illumination light of a laser beam or the like and imaging reflected light of the illumination light reflected from the measurement target, and measuring the surface state of the measurement target. For example, in the case where the measurement target is a steel plate, measurement of the surface state is performed in order to monitor surface abnormalities that may degrade product quality.
The surface abnormalities to be measured include a minute surface abnormality that may be missed in a visual inspection on a video on a monitor, and a wide and large surface abnormality that can be monitored in a visual inspection during plate passing. Here, examples of the large surface abnormality include yellowing and a temper color, and there are many abnormalities that should be distinguished from each other. On the other hand, examples of the minute surface abnormality include rough skin in which surface roughness varies and scale residue. For monitoring separately those surface abnormalities, Patent Literature 1 discloses a method of inspecting flaws occurred on a steel plate, the method including irradiating a surface of the steel plate with first strip-shaped light and second strip-shaped light, the second strip-shaped light being white light, detecting minute flaws by imaging a reflected image of the first strip-shaped light in a gray image, detecting sheet flaws by imaging a reflected image of the second strip-shaped light in a color image having resolution lower than the resolution of the gray image, and determining a type of sheet flaws using hues.